1. Field of the Invention
The present invention relates in general to survey operations in an oil and gas wellbore, and in particular relates to techniques for obtaining more accurate magnetometer data.
2. Description of the Prior Art
In order to understand the present invention, it is important to establish the definitions of terminology utilized in wellbore survey operations. Surveys are used to determine the position of a wellbore within the earth. The orientation of the wellbore is determined at a particular depth by the deviation of the wellbore from a predetermined axis. The deviations may be measured with respect to two reference vectors:
(1) the earth's gravitational field vector "G"; and PA1 (2) the earth's magnetic field vector "H". PA1 (1) an axial biasing error e.sub.z ; and PA1 (2) a transverse (or "cross-axial") biasing error e.sub.xy.
In accordance with this convention, G is positive in the vertical downward direction, while H is positive in the north direction and is inclined by a dip angle D below the horizon, as is shown in FIG. 1.
In the frame of reference established by the earth's gravitational field vector G, the deviation is identified as an inclination "I" angle, which is the angle between the longitudinal axis of the drillstring and the earth's gravitational field vector G. In the frame of reference established by the earth's magnetic field vector H, the deviation is identified as a magnetic azimuth "A", which is the angle between the horizontal component of the longitudinal axis of the drillstring and the horizontal component of the earth's magnetic field vector H.
Wellbore survey operations also frequently utilize the dip angle "D", which is the complement of the angle between the earth's magnetic field vector H and the earth's gravitational field vector G (that is, 90 degrees less the angle between vectors H and G). The dip angle D is available in look-up tables for all latitudes and longitudes on the earth's surface.
In conventional wellbore survey operations, accelerometers are utilized to measure the direction of the earth's gravitational field vector G, and magnetometers are utilized to measure the earth's magnetic field vector H. Each vector, includes x-axis, y-axis, and z-axis components.
In order to understand the techniques of the present invention for compensating for the magnetic field biasing error "e", it is important first to understand the coordinate systems utilized in surveying operations. FIG. 2 provides a view of the Cartesian coordinate system which is established relative to the bottomhole assembly of a drillstring. The z-axis of the coordinate system is in alignment with the longitudinal axis of the bottom hole assembly. The x-axis and y-axis are perpendicular to the z-axis, and are fixed in position with respect to the bottom hole assembly. Therefore, rotation of the bottom hole assembly relative to the z-axis also rotates the x-axis and the y-axis by an amount measured in terms of tool face angle "T". Note that the inclination angle I provides a measure of the position of the bottomhole assembly of a drillstring relative to the gravity field vector G.
An alternative coordinate system is frequently utilized when the earth's magnetic field is discussed. The earth's field can be approximated by a magnetic dipole, such as that shown in FIG. 3. Note that the earth's North magnetic pole is the pole towards which the north end of a compass needle is attracted: it is the south pole of the dipole. When the earth's magnetic field is discussed, it is common to employ a N-E-V (North-East-Vertical) coordinate system. By convention, the earth's magnetic field H points toward the north magnetic pole, inclined by a dip angle D below the horizon in the northern hemisphere. FIG. 4 provides a view of this coordinate system. In this coordinate system, the V-axis is aligned with the gravity field vector G, and the x-y plane is parallel with the "horizon".
During survey operations, magnetometer readings are taken along the three axes to determine the magnetic field vector H components: these measurements are identified as H.sub.x, H.sub.y, and H.sub.z. Accelerometer readings are taken along the three axes to determine the gravitational field vector G components: these measurements are identified as G.sub.x, G.sub.y, and G.sub.z. Since these vectors have both a direction and a magnitude, these tri-axial measurements will have to be scaled, or calibrated, to accurately reflect the magnitude of the vector they represent.
Survey tools utilizes these x-y-z arrays of accelerometers and magnetometers to determine the directions and magnitudes of the H and G vectors in the x-y-z frame of reference. This information is used to determine the tool's "attitude", expressed by the inclination angle I, and the azimuth angle A, as is shown in FIG. 5 and FIG. 6.
Once a set of values for H.sub.x, H.sub.y, H.sub.z, G.sub.x, G.sub.y, and G.sub.z are determined for a specific wellbore depth, the azimuth A and inclination I of the wellbore at that depth can be determined. Also, the value of the magnetic dip angle D can be calculated. The magnetic dip angle D is the complement angle between the earth's magnetic field vector and the gravitational field vector G. An expected magnetic dip angle D can be looked up in reference tables based on the wellsite's longitude and latitude. The sensors are also utilized to determine the tool face angle T. The three accelerometers are normally used to find I and G, the magnetometers are used to find H, and a combination of measurements from all sensors are required to calculate A and D.
Once the azimuth A and inclination I are determined for the wellbore at a number of specific depths, a directional map of the wellbore can be plotted. This directional map shows how far, and in what direction, the wellbore is deviated from the vertical and magnetic north, and ultimately where the well is bottomed.
In order to provide the greatest accuracy in magnetic field measurements, the wellbore surveying tool is typically housed in a non-magnetic subassembly. Additionally, surrounding subassemblies may also be constructed of a non-magnetic material. However, the drillstring, including these non-magnetic subassemblies, can nonetheless become magnetized during drilling operations, to such an extent that the accuracy of the magnetic field measurements is severely impaired. Any magnetization of the bottomhole assembly in the vicinity of the surveying equipment will introduce a biasing error "e", which is an undesired error component contained in magnetometer readings due to the magnetization of the drillstring. The biasing error includes the following two types of error components:
FIGS. 7a and 7b graphically depict the effects of an axial magnetic field bias "e.sub.z " on the determination of azimuth A. As is shown in FIGS. 7a and 7b, the effect of the axial error field e.sub.Z is to typically increase apparent azimuth A.sub.m if the hole has a westward component as is shown in FIG. 7a, or to typically decrease apparent azimuth A.sub.m if the hole is eastbound, as is shown in FIG. 7b. In either case, the apparent bottomhole location is moved further north. This is the typical case for boreholes located in the northern hemisphere. When undesired spurious magnetization is present, magnetization and accelerometer readings provide values for the measured horizontal component of the magnetic field H.sub.NEM and measured Azimuth A.sub.m, but we wish to find the unbiased horizontal component of the magnetic field H.sub.NE and especially an unbiased azimuth A.